When the electric motor of a power tool is started, there is an in-rush current that greatly exceeds the steady state current. In many power tools, this in-rush current must be compensated for to avoid problems. For example, in larger electrical motors, such as four horsepower and above, the in-rush current is sufficient to trip a 20 amp circuit breaker, the typical size circuit breaker used to protect the electrical outlets in houses. To prevent the problems caused by the in-rush current when the power tool motor is first started, the motor is soft started in many power tools.
Two approaches are typically used for soft starting electric motors in power tools—phase control, sometimes referred to as electronic, and mechanical. In the phase control approach, a thyristor, such as a triac or silicon controlled rectifier (SCR) is used to switch power to the motor. When the motor is first started, turn-on of the thyristor is delayed a predetermined number of degrees out of each half cycle of the AC waveform. For example, turn-on of the thyristor might be delayed thirty degrees during each half cycle of the AC waveform so that the thyristor is not turned on in each half cycle until thirty degrees into the half cycle. After the motor reaches a predetermined speed, or an appropriate period of time elapses, the delayed turn-on of the thyristor ceases and the thyristor is turned-on at the beginning of each half-cycle.
In the mechanical approach, a power resistor is switched in series with the motor windings during soft starting. Once the speed of the motor reaches a predetermined speed, or an appropriate period of time lapses, the power resistor is bypassed. The switch used to switch power to the motor typically includes the appropriate contacts that connects the power resistor in series with the motor windings and then bypasses the power resistor.
Both the phase control and mechanical approaches have the disadvantages of taking up space and adding cost. In addition, the phase control approach reduces motor system performance because of the power loss from phase control. The mechanical approach additionally has the disadvantage that the power resistor must be able to withstand the in-rush current which typically dictates that a ceramic power resistor be used. The ceramic power resistor must be packaged in the power tool in such a manner to protect it from shock so that it won't break if the power tool is dropped.